1. Field of the Invention
The present invention relates to a method for driving a flat-panel display device with the potential of a counter electrode to be inverted with respect to a reference potential.
2. Description of the Related Art
In general, in a liquid crystal display device, to prevent degradation of the characteristics of its liquid crystal layer, the polarity of a voltage applied across the liquid crystal layer is inverted periodically. The method of inverting the polarity of the liquid crystal application voltage for every predetermined number of frames is called “frame inversion driving”. Actually, however, flicker is caused due to asymmetry of the voltage between the positive and negative polarities. As a driving method capable of reducing the flicker, “H-line inversion driving” and “HV inversion driving” are known. In the H-line inversion driving method, the polarity of the liquid crystal application voltage is inverted for every one or more predetermined gate lines (rows). In the HV inversion driving method, the polarity of the liquid crystal application voltage is inverted for every pixel. During the H-line inversion driving or HV inversion driving method, the potential of each signal line is switched for every predetermined number of horizontal lines to a polarity positive or negative to the potential of the counter electrode, so as to invert the polarity of the liquid crystal application voltage. Assume that switching is performed for each horizontal line, for example. During one frame, signals of a polarity positive to the potential of the counter electrode are written into pixel electrodes assigned to the odd-numbered gate lines, and signals of a polarity negative to the potential of the counter electrode are written into pixel electrodes assigned to the even-numbered gate lines. During the next frame, signals of a polarity negative to the potential of the counter electrode are written into the pixel electrodes assigned to the odd-numbered gate lines, and signals of a polarity positive to the potential of the counter electrode are written into the pixel electrodes assigned to the even-numbered gate lines.
With above-mentioned methods, the polarity of the liquid crystal application voltage is inverted, and this enables reduction of flicker to be observed on the screen due to the characteristics or imperfections of pixels.
In general, a voltage of about 4V is required for driving a liquid crystal. Therefore, when the potential of the counter electrode is fixed to perform the above-mentioned polarity inversion driving method, a dynamic range of 8V and accuracy in the voltage of each polarity are required for the output of the driving circuit. This causes a problem such as an increase in the consumption of power.
In contrast, if the polarity of the counter electrode potential is simultaneously inverted to decrease the output range of the driving circuit, the power consumption can be reduced accordingly and the voltage amplitude on the video bus can also be reduced.
FIG. 11 is a timing chart for operation timings of various components obtained in a (1H/common inversion driving) case where the counter electrode potential is inverted for every horizontal line while the H-line inversion driving method is performed using one-point-at-a-time scanning. In FIG. 11, a display signal voltage on a display signal bus, a control signal (shift pulse) SPj input to a j-th analog switch ASWj, a j-th signal line potential VSj, and the counter electrode potential Vcom are arranged from top down. As shown in FIG. 11, the counter electrode potential Vcom is inverted with respect to the half value of the maximum amplitude of the signal line potential to have the maximum and minimum levels of 5V and 0V. Assume here that the liquid crystal application voltage is regarded as being of a positive polarity when the counter electrode potential Vcom is set at the minimum level, and of a negative polarity when the counter electrode potential Vcom is set at the maximum level. When the normally white display mode is used, the signal line potential of the positive polarity is set at 0.5V for providing a display state of white and at 4V for providing a display state of black, whereas the signal line potential of the negative polarity is set at 4.5V for providing a display state of white and at 1V for providing a display state of black. In this display device, transition of each signal line potential becomes to the maximum, for example when both of two adjacent horizontal lines are set into the display state of black. In this case, it is expected that the signal line potential varies from 4V to 1V, and the maximum transition becomes to 3V.
Actually, however, the counter electrode potential is inverted while the signal lines are in a floating state. Therefore, the potential of each signal line varies with the potential of the counter electrode due to coupling between the counter electrode and the signal line. As a result, the signal line potential is shifted by +5V in accordance with the counter electrode potential, and reaches 9V. The signal line holds 9V until the next display signal is written thereto. In this state, if a signal line potential of 1V is written to set two adjacent horizontal lines at the black level, the signal line potential shifts at a variation range of 8V due to the inversion of the counter electrode potential.
As described above, in the case where H/common inversion driving is performed in the conventional liquid crystal display device, a variation in potential occurs in each signal line when the counter electrode potential is inverted for each predetermined horizontal line and frame, which increases the variation range of the signal line potential for the next writing operation. For example, the closer to black the display color of display pixels adjacent in a row direction, the greater the variation of the signal line potential and hence the higher the possibility of a defective display by the influence of the potential variation in each signal line.